Encapsulation and controlled delivery of strong mineral acids

ABSTRACT

A polymer-encapsulated mineral acid solution and a method for forming the polymer-encapsulated mineral acid solution. Introducing a strong mineral acid solution to a monomer solution occurs such that a primary emulsion that is a water-in-oil type emulsion forms. Introducing the primary emulsion to a second aqueous solution forms a secondary emulsion that is a water-in-oil-in-water type double emulsion. The monomer in the secondary emulsion is cured such a polymerized shell forms that encapsulates the strong mineral acid solution and forms the capsule. The strong mineral acid solution has up to 30 wt. % strong mineral acid. A method of stimulating a hydrocarbon-bearing formation using the polymer-encapsulated mineral acid solution includes introducing a capsule suspension into a fissure in the hydrocarbon-bearing formation to be stimulated through a face in a well bore. The capsule is maintained within the fissure until the polymer shell degrades.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of invention relates to the stimulation of oil and gas wells.More specifically, the field relates to the use of encapsulated acidsfor stimulating oil and gas wells.

2. Description of the Related Art

For carbonate hydrocarbon-bearing formations having fractures, fissuresand other natural and man-made hydrocarbon conduits through theformation, stimulation by conventional fracture acid treatments (strongmineral acids like HCl, H₂SO₄, HF, and HNO₃) have experiencedchallenges. The significant problem with directing conventional fractureacid treatments though such fractures or fissures to improve productionand hydrocarbon fluid flow is due to the reactivity of the acid itselfwith carbonates. Unless the carbonate has been passivized or coated witha neutral material, the acid or solution of acid reacts with the portionof the hydrocarbon-bearing formation that it immediately contacts: theportion proximate to the wellbore. Reacting close to the face or wall ofthe wellbore achieves little to no fracture conductivity improvementalong the entire fracture length or through the hydrocarbon-bearingformation.

Traditional methods to retard or delay the reaction of strong mineralacids include acid gelation, acid-in-oil (W/O) emulsification andadsorption of surfactants on the rock face. Each of these techniques haslimitations. The acid-in-oil emulsification and the acid gelation existin a metastable/unstable state that may easily break down without anycontrolled mechanism of release. Other limitations include reduction inacid efficiency; increase in cost and complexity due to the additives,especially surfactants; poor control over penetration depth thought heformation; and the need to use corrosion inhibitors as some of theadditives attach the metals of the casing and well tools.

Acid fracturing with an acid-in-oil (W/O) emulsification is a techniquewhere the strong mineral acid is surrounded by a hydrocarbon liquid suchas diesel. The diesel provides a liquid hydrophobic barrier that uponcontact with a sharp or hydrocarbon-bearing surface, or uponintroduction of an emulsion breaker, would permit the acid solution tobe released. Once the emulsion breaks the strong mineral acid is able toreact with the carbonate rock. The two main problems with this techniqueare that there is a lack of control over the acid release and thequestionable shear resistance of the emulsion itself. The result of suchan application is a less localized, moderately deeper into the formationbut still unevenly distributed fracture conductivity improvement.

Other alternatives include the use of weaker organic acids, includingcitric acid. The results have been mixed. In general, weaker organicacids exhibit a significantly lower bulk dissolution capacity thanstrong mineral acids. They are generally more expensive per equivalentacid volume. Organic acids, however, do demonstrate an ability toperform such that there is greater etched fissure conductivity than withtraditional mineral acid application. It is noted that some care must betaken with the use of organic acids utilized downhole in the balancebetween etch properties and precipitation. It is understood that incarbonate formations that the increase in released calcium ions duringcarbonate rock dissolution can cause secondary precipitation, which canthen end up clogging up equipment or formation pores that were beingattempted to clear. Note that the combination of strong mineral acidswith organic acids is used in wells with special types of tubing.

There is a need for the downhole application of a simpler acidapplication system that applies the power of strong mineral acids in away that achieves the precision etching of organic acids. Such a systemshould be easy to use and improve etched fissure conductivity overconventional fracture acid treatments.

SUMMARY OF THE INVENTION

A method for forming a polymer-encapsulated mineral acid solutionincludes introducing a strong mineral acid solution to a monomersolution. The strong mineral acid solution comprises a strong mineralacid that is in a range of from greater than 0 wt. % to 30 wt. % of thestrong mineral acid solution. The monomer solution comprises a monomerand a free-radical initiator. The monomer is hydrophobic. The monomer isalso operable to polymerize upon initiation of a free-radical chainpolymerization reaction. The introduction occurs such that a primaryemulsion forms. The primary emulsion is a water-in-oil (W/O) typeemulsion. The method also includes introducing the primary emulsion to asecond aqueous solution. A secondary emulsion forms. The secondaryemulsion is a water-in-oil-in-water (W/O/W) type double emulsion. Themethod includes converting the free-radical initiator in the monomersolution. A free-radical chain polymerization reaction is initiated inthe monomer solution. The polymerized shell forms. The polymerized shellencapsulates and prevents interaction with the strong mineral acidsolution until the polymerized shell degrades. The strong mineral acidsolution does not degrade the polymerized shell. The secondary emulsionconverts into the polymer-encapsulated mineral acid solution.

The polymer-encapsulated mineral acid solution includes the strongmineral acid solution. The strong mineral acid is in the range of fromgreater than 0 wt. % to about 30 wt. % of the solution. The capsule alsoincludes the polymerized shell that encapsulates the strong mineral acidsolution. The polymerized shell encapsulates and prevents interactionwith the strong mineral acid solution until the polymerized shelldegrades. The strong mineral acid solution does not degrade thepolymerized shell. In an embodiment of the capsules, the polymer shellhas a degradation period of about 2 hours at 350° F. In an embodiment ofthe capsules, the polymer shell has a degradation period of about 4hours at 300° F. In an embodiment of the capsules, the polymer shell hasa degradation period is about 8 hours at 275° F.

A polymer-encapsulated mineral acid solution suspension includes apolymer-encapsulated mineral acid solution and a suspension fluid. In anembodiment of the suspension, the suspension fluid is artificial brine.An artificial brine comprising NaCl and CaCl₂ is useful. In anotherembodiment of the suspension, the suspension fluid is produced formationwater. The total dissolved solids in the suspension may be in a range offrom greater than 0 to about 40,000 parts-per-million (ppm). In anembodiment, the suspension fluid further comprises an acidic compound.

A method of stimulating a hydrocarbon-bearing formation using apolymer-encapsulated mineral acid solution includes introducing thepolymer-encapsulated mineral acid solution suspension into a fissure inthe hydrocarbon-bearing formation to be stimulated through a face in awell bore. The well bore is defined by a well bore wall and traversesthe hydrocarbon-bearing formation. The face is a portion of the wellbore wall associated with and operable to provide fluid communicationbetween the hydrocarbon-bearing formation and the well bore. The fissurein the hydrocarbon-bearing formation is accessible through the face. Themethod also includes maintaining the polymer-encapsulated mineral acidsolution within the fissure. The polymer shell degrades within thefissure where the strong mineral acid solution is released into thefissure of the hydrocarbon-bearing formation. Upon interaction betweenthe strong mineral acid and the hydrocarbon-bearing formation, thehydrocarbon-bearing formation is stimulated.

The capsules have a liquid core/polymer shell form that encapsulates thestrong mineral acid solution. The capsules have a mean diameter rangeanywhere from the micrometer (μm) range to the nanometer (nm) range.

The core is filled with the liquid acid solution while the coating(shell) is a polymer-based material. The encapsulated strong mineralacid solution offers an attractive alternative means to achieve acontinuously conductive fracture plane, with enhanced differentialrelief, by delivering the microparticles to nanoparticles along thefractures and fissures of the hydrocarbon-bearing formation beforerelease. Slow, continuous releasing of the strong mineral acid solutionby capsules of various designs can enable the acid etchant to diffuseslowly along the fissure and fracture conduits through the formation,increasing the effectiveness of the stimulation and resulting inimproved mobility. The repeated minor acid exposures as the capsulesdegrade within the fractures and fissure permits the microparticles andnanoparticles of encapsulated strong mineral acid to travel further andform increased fracture length pathways through the formation thantraditional acid treatments.

The polymer shell has tunable degradation profiles. The polymer shelllacks permeability initially upon introduction. The polymer shell servesas a barrier to minimize contact between the strong mineral acid and thecarbonate rock such that the acid can be delivered deeper into theformation along fractures and fissures. The polymer shell is designed todegrade over a period of hours, days or weeks within ahydrocarbon-bearing formation or well bore depending on its intendeduse.

The polymer-encapsulated mineral acid solution enable placement ofstrong mineral acids deeper into the reservoir as compared totraditional hydrocarbon-emulsified acids or bare mineral or organicacids. The polymer-encapsulated mineral acid solution permit delayed anddesigned release of the strong mineral acid into carbonate reservoirs,which permits the acid to be delivered much deeper than it would bepermitted in either bare or emulsified forms. The polymer-encapsulatedmineral acid solution reduce or eliminate the contact of the strongmineral acid with injection tubing, downhole tools and casing, whichmitigates corrosion due to acid exposure.

The method of forming the polymer-encapsulated mineral acid solution ischallenging due to the requirement of the polymerization reaction tooccur in an environment proximate to or within a strong mineral acidsolution, which can prevent or accelerate the polymerization of themonomer. Certain polymerization processes, including free-radical chainpolymerization, appear not to be effected by the proximity of the strongmineral acid solution and the polymerization reaction process in anadjacent liquid layer. Free-radical chain polymerization can utilize anumber of hydrophobic monomers, which is a property that permits themonomer solution layer to remain separate from the strong mineral acidsolution layer, such that the monomer solution layer forms theencapsulating polymer shell around the strong mineral acid solution.Balancing both inner and continuous aqueous layers of the W/O/W doubleemulation during formation permits capsules with strong mineral acidsolution cores having a concentration of strong mineral acid in a rangeof from greater than 0 wt. % to about 30 wt. %—greater than provided inprevious references and with materials traditionally used forencapsulation.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention are better understood with regard to the following DetailedDescription of the Preferred Embodiments, appended Claims, andaccompanying Figures, where:

FIG. 1 is a simplified diagram of a polymer-encapsulated mineral acidsolution;

FIG. 2 is a process flow diagram of an embodiment of a system forcontinuously performing an embodiment of the water-in-oil-in-water(W/O/W) double-emulsion process to form the product capsules;

FIG. 3 is a graph showing traces for mass versus temperature of theseveral acrylate polymers;

FIG. 4 is a graph showing traces for storage modulus of the severalacrylate polymers;

FIG. 5 is a graph showing traces for tan delta of the several acrylatepolymers;

FIG. 6 is a graph showing traces for storage modulus of diurethanedimethacrylate polymer after no immersion and 24 hours of immersion in a20 wt. % HCl solution;

FIG. 7 is a graph showing traces for tan delta of Bisphenol-Aglycerolate methacryalte polymer after no immersion and 24 hours ofimmersion in a 20 wt. % HCl solution;

FIG. 8 is a graph showing traces for determined storage modulus ofdiurethane dimethacrylate after no immersion, 24 hours of immersion and2 weeks of immersion in a 20 wt. % HCl solution;

FIG. 9 is a graph showing traces for determined tan delta of diurethanedimethacrylate after no immersion, 24 hours of immersion and 2 weeks ofimmersion in a 20 wt. % HCl solution;

FIGS. 10 a-c are scanning electron micrographs (SEMs) of synthesizedpolymer-encapsulated mineral acid solution having 10 wt. % HCl core;

FIGS. 11 a-c are SEMs of synthesized polymer-encapsulated mineral acidsolution having 20 wt. % HCl core;

FIGS. 12 a-b are optical microscopy images of the synthesizedpolymer-encapsulated mineral acid solution having 20 wt. % HCl core;

FIGS. 13 a-c are SEMs of synthesized polymer-encapsulated mineral acidsolution having 30 wt. % HCl core;

FIGS. 14 a-b are optical microscopy images of the synthesizedpolymer-encapsulated mineral acid solution having 30 wt. % HCl core;

FIG. 15 is a SEM of the synthesized polymer-encapsulated mineral acidsolution having 20 wt. % HCl core and a poly(PEG dimethacrylate) shell;

FIGS. 16 a-b are SEMs of the synthesized polymer-encapsulated mineralacid solution having 20 wt. % HCl core and a poly(PEG dimethacrylate)shell after exposure to brine for 24 hours at 25° C.;

FIG. 17 is an optical microscopy image of the synthesized 1,6-hexanedioldiacrylate polymer-encapsulated mineral acid solution having 20 wt. %HCl core;

FIG. 18 is a graph showing traces for determined average pH versus timefor 20 wt. % HCl core/poly(1,6-hexanediol diacrylate) shell capsules atless than and greater than T_(g) temperatures;

FIG. 19 is a SEM of synthesized polymer-encapsulated mineral acidsolution formed in an osmotically unbalanced system;

FIG. 20 is a SEM of synthesized polymer-encapsulated mineral acidsolution formed in osmotically balanced system (NaCl/NaCl);

FIG. 21 is a SEM of synthesized polymer-encapsulated mineral acidsolution formed in osmotically balanced system (NaCl/HCl);

FIG. 22 is a graph showing traces for determined change in average pHversus time for osmotically-balanced HCl [5.7M] core/poly(1,6-hexanedioldiacrylate) shell capsules below and above-T_(g) temperature and forsimilarly osmotically-balanced NaCl [5.7M] capsules;

FIGS. 23 a-c are optical microscopy images of the synthesized epoxypolymer-encapsulated mineral acid solution having 1 wt. % HCl cores;

FIGS. 24 a-b are SEMs of synthesized epoxy polymer-encapsulated mineralacid solution and resultant salts;

FIGS. 25 a-b are SEMs of the synthesized epoxy polymer-encapsulatedneutral water; and

FIGS. 26 a-b are SEMs of resultant salts from attempting to synthesizeepoxy polymer-encapsulated mineral acid solution.

FIGS. 1-26 and their description facilitate a better understanding ofand method of use for use of the polymer-encapsulated mineral acidsolution and the system and method for forming polymer-encapsulatedmineral acid solution. In no way should FIGS. 1-26 limit or define thescope of the invention. FIGS. 1 and 2 are simple diagrams for ease ofdescription.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The Specification, which includes the Summary of Invention, BriefDescription of the Drawings and the Detailed Description of thePreferred Embodiments, and the appended Claims refer to particularfeatures (including process or method steps) of the invention. Those ofskill in the art understand that the invention includes all possiblecombinations and uses of particular features described in theSpecification. Those of skill in the art understand that the inventionis not limited to or by the description of embodiments given in theSpecification. The inventive subject matter is not restricted exceptonly in the spirit of the Specification and appended Claims.

Those of skill in the art also understand that the terminology used fordescribing particular embodiments does not limit the scope or breadth ofthe invention. In interpreting the Specification and appended Claims,all terms should be interpreted in the broadest possible mannerconsistent within the context of each term. All technical and scientificterms used in the Specification and appended Claims have the samemeaning as commonly understood by one of ordinary skill in the art towhich the invention belongs unless defined otherwise.

As used in the Specification and appended Claims, the singular forms“a”, “an” and “the” include plural references unless the context clearlyindicates otherwise. The verb “comprises” and its conjugated formsshould be interpreted as referring to elements, components or steps in anon-exclusive manner, and the invention illustrative disclosed suitablymay be practiced in the absence of any element which is not specificallydisclosed, including as “consisting essentially of” and “consisting of.The referenced elements, components or steps may be present, utilized orcombined with other elements, components or steps not expresslyreferenced. The verb “couple” and its conjugated forms means to completeany type of required junction, including electrical, mechanical orfluid, to form a singular object from two or more previously non-joinedobjects. If a first device couples to a second device, the connectioncan occur either directly or through a common connector. “Optionally”and its various forms means that the subsequently described event orcircumstance may or may not occur. The description includes instanceswhere the event or circumstance occurs and instances where it does notoccur. “Operable” and its various forms means fit for its properfunctioning and able to be used for its intended use. “Associated” andits various forms means something connected with something else becausethey occur together or that one produces the other. “Detect” and itsconjugated forms should be interpreted to mean the identification of thepresence or existence of a characteristic or property. “Determine” andits conjugated forms should be interpreted to mean the ascertainment orestablishment through analysis or calculation of a characteristic orproperty. “Fluids” means vapors, liquids, gases and their combinationsat their present condition unless otherwise stated.

Spatial terms describe the relative position of an object or a group ofobjects relative to another object or group of objects. The spatialrelationships apply along vertical and horizontal axes. Orientation andrelational words are for descriptive convenience and are not limitingunless otherwise indicated.

Where the Specification or the appended Claims provide a range ofvalues, it is understood that the interval encompasses each interveningvalue between the first limit and the second limit as well as the firstlimit and the second limit. The invention encompasses and bounds smallerranges of the interval subject to any specific exclusion provided.“Substantial” means equal to or greater than 10% by the indicated unitof measure. “Significant” means equal to or greater than 1% by theindicated unit of measure.

Where the Specification and appended Claims reference a methodcomprising two or more defined steps, the defined steps can be carriedout in any order or simultaneously except where the context excludesthat possibility.

FIG. 1

FIG. 1 is a simplified diagram of a polymer-encapsulated mineral acidsolution. Capsule 100 has interior 102 filled with an aqueous strongmineral acid solution. The acid in the strong mineral acid solution isin a range of from greater than 0 wt. % to about 30 wt. % of thesolution. In an embodiment of the capsule, the acid in the strongmineral acid solution comprises hydrochloric acid (HCl).

Polymer shell 104 completely encompasses and contacts the aqueous strongmineral acid solution at inner surface 106. Capsule 100 also hasexterior surface 108 that is also defined by polymer shell 104. Polymershell 104 has thickness 110 as the radial distance between inner surface106 and exterior surface 108. Thickness 110 is one of several variablesthat can assist in determining the time to degradation for polymer shell104 and release of the aqueous strong mineral acid solution frominterior 102. Capsule 100 also has a diameter 112, which is useful indetermining the estimated depth that capsule 100 can traverse inside aformation, fissure or fracture to deliver the aqueous strong mineralacid solution.

The polymer shell is comprised of one or more monomers that haveundergone polymerization such that the polymer shell forms andencapsulates the strong mineral acid solution. A polymer made from thepolymerization of one monomer is a “homopolymer”; made from two monomersis a “copolymer”; made from three monomers is a “terpolymer”; and so on.In an embodiment of the capsule, the polymer of the polymer shell is ahomopolymer. In an embodiment of the capsule, the polymer of the polymershell is a copolymer. For co-polymers and higher order polymers, theorganization of monomers within and along the polymer chain includealternating, periodic, statistical, random, gradient, block (di-, tri-,etc.) and graft, among others.

“End groups” are the functional units that are at the extremity of amacromolecule or oligomer, such as a monomer. Useful monomers forforming the polymer shell include monomers having acrylate functionalend groups, including 1,6-hexanediol diacrylate;1,1,1-trimethylolpropane triacrylate;2,2-bis[4-(2-acryloxyethoxy)phenyl]propane; dipentaerythritolpentaacryalate; 1,1,1-trimethylolpropane triacrylate (TPT); and aurethane-acrylate oligomer such as the one that goes by the trade name“CN9013” (Sartomer Co., Inc.; Exton, Pa.). In an embodiment of thecapsule, the polymerized shell comprises polymerized monomers havingacrylate end group functionality. Useful monomers for forming thepolymer shell also include monomers having methacrylate functional endgroups, including diurethane dimethacrylate; Bisphenol-A glycerolatedimethacrylate; poly(ethylene glycol)dimethacrylate (having an averagePEG M_(n) of about 600); and 1,6-hexanediol dimethacrylate. In anembodiment of the capsule, the polymerized shell comprises polymerizedmonomers having methacrylate end group functionality. In an embodimentof the capsule, polymerized shell comprises polymerized monomersselected from the group consisting of monomer has acrylate functionalend groups, methacrylate functional end groups and combinations thereof.

The interior or “backbone” structure of each monomer (the portion of theoligomer between the functional end groups) can be chosen from a widevariety of chemical groups to help tune the properties of the formedpolymer shell (whereas end-group functionality tends to dominate thebehavior of monomers). The polymer shell may comprise, among otherthings, linear carbon chains, phenolic constructs; urethanes;poly(ethylene glycol) (PEG); ester and ether segments; andorganohalogens. The properties of the polymer shell therefore can betuned to control the strong mineral acid solution release profile,impart strength to the shell and mobility of the capsule.

To encapsulate the strong mineral acid solution into a polymer shell,the monomer used to form the polymer shell has hydrophobicity. Thispermits a monomer solution layer to form on top of a core of strongmineral acid solution and encapsulate it upon polymerization. If themonomer solution is insufficiently hydrophobic, the monomer may begin toincorporate into the strong mineral acid solution and not fully form anencapsulating layer. Also, the monomer during the double-emulsion mayalso be partially removed upon application of the second aqueoussolution (the continuous phase). The hydrophobicity of the monomersolution also permits the monomer before polymerization initiation todistribute itself more evenly around the surrounded core of strongmineral acid solution, forming a more spherical capsule.

In an embodiment of the capsule, the polymer shell is the resultant offree-radical chain polymerization reaction. The strong mineral acidsolution does not complicate or interfere with the mechanism offree-radical chain polymerization between monomer molecules. Otherpolymerization methods that are not interfered with by the presence ofthe strong mineral acid solution are also available for forming thepolymer shell.

In some embodiments of the capsule, the polymerized shell has a glasstransition temperature (T_(g)) and the T_(g) is in a range of from 43°C. to 151° C. In some embodiments of the capsule, the polymerized shelldoes not have a glass transition temperature (T_(g)).

Certain polymerization methods do not appear to work in the presence ofthe strong mineral acid solution. Step growth polymerization, such asused by monomers with epoxy functional end groups to form epoxy-basedpolymers, does not appear to provide the polymerization mechanism thattolerates the strong mineral acid solution contacting the hydrophobicmonomer solution layer. Epoxy functional end group monomers typicallyundergo step-growth polymerization in forming their associated polymer.In an embodiment of the capsule, the monomer does not have epoxyfunctional end groups. Because of the presence of a strong mineral acidthat the monomer solution layer directly contacts during formation, amonomer that is susceptible to initiation of the polymerization reactionby proton-donation (that is, acid catalysis) at the mixing conditionwhere the strong mineral acid solution and the monomer solution arefirst emulsified would prove highly ineffective in forming capsules. Themonomer would immediately react upon contact with the strong mineralacid and would not form the polymer shell; rather, loose and distributedpolymers and oligomers would randomly form as the monomer solutioninteracts with the surface of the strong mineral acid solution. As well,a monomer material that happens to react with the strong mineral acidsolution and form a salt complex would also be an ineffective material.

FIG. 2

FIG. 2 is a process flow diagram of an embodiment of a system forcontinuously performing an embodiment of the water-in-oil-in-water(W/O/W) double emulsion process to form the product capsules. System 200includes first mixer 202 with mixing apparatus 204 for forming awater-in-oil emulsion (W/O), second mixer 206 with mixing apparatus 208for forming a W/O/W emulsion using the W/O emulsion from first mixer202, polymerization initiator 210 and solids/liquids separator 212.

Feeds to system 200 include acid feed line 220, which introduces thestrong mineral acid solution into first mixer 202, monomer feed line222, which introduces the monomer that is associated with the polymershell that encapsulates the strong mineral acid solution, and water feedline 224, which introduces the secondary aqueous solution that forms theexterior emulsification agent for the W/O/W double emulsion.

Products from system 200 include the polymer-encapsulated mineral acidsolution, which are produced through product line 230 as a dispersion orslurry of solid capsules in a portion of post-activated W/O/W doubleemulsion fluid. Waste fluid line 232 conveys from solids/liquidsseparator 212 the remaining post-activated W/O/W double emulsion fluid,which includes unused aqueous fluids, including strong mineral acidsolution, unreacted monomer and associated polymers and oligomers of themonomer.

The strong mineral acid solution is introduced into first mixer 202through acid feed line 220. The strong mineral acid solution comprises astrong mineral acid. In an embodiment of the method, the strong mineralacid is selected from the group consisting of hydrochloric acid (HCl),hydrofluoric acid (HF), sulfuric acid (H₂SO₄), nitric acid (HNO₃) andcombinations thereof. The strong mineral acid solution is thediscontinuous or minority phase of the primary emulsification thatoccurs in first mixer 202 that forms the core material of the productpolymer-encapsulated mineral acid solution. In an embodiment of themethod, the strong mineral acid solution further comprises afree-radical initiator.

The monomer solution is introduced into first mixer 202 through monomerfeed line 222. The monomer used is a liquid hydrophobic monomer beforeit is polymerized into the polymer shell. In an embodiment of themethod, the monomer solution comprises a monomer having acrylate endgroup functionality. In an embodiment of the method, the monomersolution comprises a monomer having methacrylate end groupfunctionality. The monomer solution dissolves and contains othercomponents useful for polymerizing the monomer as well as balancingproperties of the monomer solution. In such embodiments, the monomersolution may further comprise a hydrocarbon, including a hydrocarbonfraction such as diesel, or a refined or purified hydrocarbon, such asbenzene and the xylenes. In an embodiment of the method, the monomersolution further comprises a free-radical initiator. Several knownradical initiators are soluble in the hydrocarbon or monomer phase andnot soluble in an aqueous or water phase. In an embodiment of themethod, the monomer solution further comprises a second monomer. Themonomer solution is the continuous or majority phase of the primaryemulsification that occurs in first mixer 202 that forms the polymershell of the product polymer-encapsulated mineral acid solution.

When emulsifying the primary emulsion into a secondary emulsion, thereis a concern regarding the sheer force applied to the second interiorphase (the monomer solution) while it is interacting with the continuousphase. There is also a concern regarding the application of momentum tothe W/O combination. If both fluids of the primary emulsion hasdifferent densities, the application of force to the W/O emulsion withina second continuous fluid will give the first aqueous phase and the oilphase different moments of momentum. During prolonged emulsification,eventually the differences in density can cause the components of theprimary emulsion to physically pull apart separate. Given that thepolymer shell has not formed, there is no barrier other than therepulsion to the hydrophobic monomer that surrounds the strong mineralacid solution unless another material, including a hydrocarbon fluid, isincorporated into the monomer solution. The density of the monomersolution can be raised by adding a solvent or a co-monomer, including1,6-hexanediol diacrylate. The density and viscosity of the strongmineral acid solution can be elevated by adding thickening agents thatare also weighting agents, including polyvinyl alcohol (PVA). In anembodiment of the method, the monomer solution has a density that is notsubstantially different than the density of the strong mineral acidsolution. In an embodiment of the method, the difference is notsignificant. In an embodiment of the method, the monomer solution has aviscosity that is not substantially different than the viscosity of thestrong mineral acid solution.

In first mixer 202 the introduced strong mineral acid solution isemulsified into the monomer solution using mixing apparatus 204, formingthe water-in-oil (W/O) emulsion. System 200 passes the W/O emulsion fromfirst mixer 202 using second reactor feed line 240.

The secondary aqueous solution is introduced into second mixer 202through water feed line 220. The secondary aqueous solution is thecontinuous or majority phase of the secondary emulsification that occursin second mixer 206. The secondary aqueous solution causes thehydrophobic monomer solution of the oil phase of the W/O emulsion to betrapped between two layers of aqueous fluid: the strong mineral acidsolution and the secondary aqueous solution. In an embodiment of themethod, the secondary aqueous solution further comprises a free-radicalinitiator.

In second mixer 206 the introduced W/O emulsion is further emulsifiedinto the secondary aqueous solution using mixing apparatus 208, formingthe water-in-oil-in-water (W/O/W) emulsion. System 200 passes the W/O/Wdouble emulsion from second mixer 206 using polymerization initiatorfeed line 242.

With greater concentrations of mineral acid used in forming the core ofthe polymer-encapsulated mineral acid solution (and in the internalwater phase) and the monomer solution phase forming a semi-permeablebarrier, osmotic pressure can build between the two water phases (theinternal water phase and the continuous water phase) such that waterfrom the continuous phase attempts to communicate with and dilute theinternal phase through the monomer solution phase. Because of itshydrophobic nature and the monomer has not formed the polymer shell atthis point of the process, the monomer solution phase can act as asemi-permeable membrane to the two aqueous phases. If the osmoticpressure is high enough, the water from the continuous phase can pushinto the monomer solution phase flow into the aqueous interior phase.Adjusting the composition of the secondary aqueous solution such thatthe osmotic pressure is diffused can prevent osmosis from occurring canpreserve the integrity of the hydrophobic monomer solution and theefficacy of the internal water phase. An acid may be used to adjust thesecondary aqueous solution composition. In an embodiment of the method,the secondary aqueous solution further comprises an acid. In anembodiment of the method, the acid dissociates a chlorine anion. In suchembodiments, the acid is hydrochloric acid (HCl). In such embodiments,the concentration of hydrochloric acid in the secondary aqueous solutionis in a range of from about 1 wt. % to 20 wt. % of the solution. The twoaqueous water phases do not have to have equal concentrations of ions tooff-set one another. The two aqueous water phases can remain imbalancedthrough the process such that that the osmotic pressure of thecontinuous water phase cannot overcome the monomer solution phase. In anembodiment of the method, the secondary aqueous solution has a negativeion concentration that is not substantially different than the negativeion concentration of the strong mineral acid solution. In an embodimentof the method, the difference is not significant. In an embodiment ofthe method, the concentrations are equal. A salt may also be used toadjust the secondary aqueous solution composition. In an embodiment ofthe method, the secondary aqueous solution further comprises a salt. Inan embodiment of the method, the secondary aqueous solution furthercomprises a salt that dissociates a chlorine anion. In such anembodiment, the salt is sodium chloride (NaCl). In an embodiment of themethod, the secondary aqueous solution has a negative anionconcentration that is not substantially different than the negative ionconcentration of the strong mineral acid solution. In an embodiment ofthe method, the difference is not significant. In an embodiment of themethod, the concentrations are equal.

The W/O/W double emulsion passing from second mixer 202 is still anemulsion of three separate liquids. The hydrophobic monomer solution inthe “oil” phase has not been cured. System 200 passes the W/O/W doubleemulsion product from second mixer 206 via polymerization initiator feedline 242 to polymerization initiator 210. Polymerization initiator 210is operable to initiate a free-radical chain polymerization in themonomer solution of each W/O/W emulsion by converting the free-radicalinitiator into free radicals, which initiate the radical polymerizationreaction of the monomer of the monomer solution phase. The monomerreacts and forms a solid polymer layer where the monomer solution ispresent.

The cure rate within the polymerization initiator and the conversion ofthe free-radical initiator is such that the monomer solution phasesurrounding the strong mineral acid solution phase forms the polymershell that encapsulates the strong mineral acid solution. The cure rateof the polymer is such that the polymer shell develops a thicknessmeasured radially from the interior surface to the exterior surface ofthe polymer-encapsulated mineral acid solution. The solid near-sphericalor spherical polymer layer is the polymer shell that encapsulates thestrong mineral acid solution.

In an embodiment of the method, converting the free-radical initiator inthe monomer solution occurs by introducing electromagnetic energy intothe secondary emulsion. In such embodiments, the free-radical initiatoris operable to form free radical species upon adsorbing introducedelectromagnetic energy. Polymerization initiator is operable tointroduce the electromagnetic energy into the W/O/W double emulsion.Introduced electromagnetic energy can include infra-red (IR) waves,visible light, including natural, filtered, polarized and artificiallight, ultraviolet light, X-rays, gamma rays, radio waves andmicrowaves. In an embodiment of the method, the free-radical initiatoris a photo-initiator. Ultraviolet (UV) light is typically used to breakapart the free-radical initiators. 2,2′-Azobis(2-methylpropionitrile)(AIBN), 2-hydroxy-2-methylpropiophenone,diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (DPO), benzoyl peroxide(BPO) and blends thereof are well-known photo-initiator (light-activatedradical generators).

In an embodiment of the method, converting the free-radical initiator inthe monomer solution occurs by introducing heat into the secondaryemulsion. In such embodiments, the free-radical initiator is operable toform free radical species upon raising the temperature to a temperaturespecific to the initiator. The polymerization initiator introducing heatcan indirectly convey heat to the W/O/W double emulsion, such as througha heat exchanger, or directly by introducing a stream to incorporatewith the double emulsion and directly convey heat into it. Di-tert-butylperoxide (DTBP), which has low solubility in water, is known as aninitiator that breaks down at elevated temperatures. AIBN and BPO arealso have low solubility in water and are thermal initiators.

Upon polymerization of the monomer and formation of thepolymer-encapsulated mineral acid solution, the W/O/W double emulsioncomposition is converted into the post-activated W/O/W double emulsionfluid, which is effectively a raw product. The post-activated W/O/Wdouble emulsion fluid contains the product capsules, unused aqueousfluids, including strong mineral acid solution, unreacted monomer andassociated polymers and oligomers of the monomer. System 200 passes theraw product stream through separator feed line 246 into solids/liquidsseparator 212. Solids/liquids separator is operable to separate thesolids, including heavy oligomers, unsuspended polymers and the productpolymer-encapsulated mineral acid solution from the liquids, includingunreacted monomer, oligomers, suspended polymers and aqueous solution,of the introduced post-activated W/O/W double emulsion fluid.

Experiments

Examples of specific embodiments facilitate a better understanding ofthe formation and use of polymer-encapsulated mineral acid solution. Inno way should the Examples limit or define the scope of the invention.

Characterization of Shell Stability—Acrylate Polymers

Several acrylate and methacrylate-based polymers are examined forunderstanding their basic properties and how they may be applicable fordownhole service in an elevated temperature/brine/acid environment.

Several homopolymers are made from the following monomers: diurethanedimethacrylate; ethoxylated trimethyolpropane triacrylate; Bisphenol-Aglycerolate dimethacrylate; 1,6-hexanediol diacrylate (Polysciences#23671-100) (Polysciences, Inc.; Warrington, Pa.); and SATOMER urethaneacrylate oligomer (CN9013). Diurethane dimethacrylate includes a mixtureof isomer monomers where the diurethane has a hydrogen:methane ratio ofabout 1:1 (Sigma-Aldrich #436909) (Sigma-Aldrich Corp.: St. Louis, Mo.).In addition, a copolymer is made: 2-hydroxylethylacrylate/2,2-Bis[4-(2-acryloxyethoxy)phenyl]propane. The polymerizationreaction was initiated using UV light with 2 wt. % of2-hydroxy-2-methylpropiophenone, which is a photo-initiator.

The resultant polymers are each examined using a dynamic mechanicalanalysis (DMA) testing machine and a thermogravimetric analysis (TGA)analyzer. The DMA testing provides information regarding glasstemperature transition (T_(g)), which is the temperature where thepolymer transitions from a hard, brittle state to a rubbery, elastomericstate. It also provides information on the modulus of the polymer, whichis the tendency to deform under a constant applied force. TGA testingprovides the thermal stability of the tested material, includingproviding a decomposition temperature (T_(d)).

FIG. 3 is a graph showing traces for TGA (mass versus temperature) ofthe several acrylate polymers. Table 1 shows the determined T_(d) in °C. for the several acrylate polymers.

TABLE 1 Onset of decomposition (T_(d)) for several acrylate polymers.Acrylate System T_(d) (° C.) diurethane dimethacrylate 305 ethoxylatedtrimethyolpropane triacrylate 370 Bisphenol-A glycerolate dimethacrylate367 1,6-hexanediol diacrylate 380 2-hydroxylethyl acrylate/2,2-Bis[4-(2-337 acryloxyethoxy)phenyl] propane urethane acrylate oligomer (SATOMER344 CN9013)

FIG. 4 is a graph showing traces for storage modulus (DMA) of theseveral acrylate polymers. FIG. 5 is a graph showing traces for tandelta (DMA) of the several acrylate polymers. Table 2 shows thedetermined glass transition temperature (T_(g)) and storage modulus at35° C. for each of the several acrylate polymers.

TABLE 2 Glass transition temperatures (T_(g)) and storage modulus forseveral acrylate polymers. Storage Modulus at 35° C. Acrylate SystemT_(g) (° C.) (MPa) urethane acrylate oligomer — 3483 (SARTOMER CN9013)Bisphenol-A glycerolate 151 3617 Dimethacryalte diurethanedimethacrylate 149 2971 (Sigma #436909)

All of the acrylate polymers show high stability by having a greaterthan 300° C. temperature of degradation, glass transition temperatureswell above 100° C. and storage modulus greater than about 2900 MPa.

The three polymers shown in Table 3 are immersed in a 20 wt. % HClaqueous solution for 24 hours and then retested for storage modulus andtan delta to determine if there is any modification to the polymer aftera meaningful period of exposure. FIG. 6 is a graph showing traces forstorage modulus of diurethane dimethacrylate polymer after no exposureand 24 hour exposure to a 20 wt. % HCl solution. FIG. 7 is a graphshowing traces for tan delta of Bisphenol-A glycerolate methacryaltepolymer after no exposure and 24 hour exposure to a 20 wt. % HClsolution. Table 3 shows the comparative storage modulus and tan deltaresults with no and 24 hour exposure.

TABLE 3 Mechanical properties with and without exposure to 20% HCl.Storage Modulus at 35° C. (MPa) T_(g) (° C.) Immersion Immersion No HClin 20% No HCl in 20% Immer- HCl for 24 Immer- HCl for 24 Acrylate Systemsion hours sion hours Diurethane 2971 3087 149 150 Dimethacrylate (Sigma#436909) Bisphenol-A 3617 3697 151 148 Glycerolate Methacryalte UrethaneAcrylate 3483 3393 — — (Sartomer CN9013)

The mechanical properties of the three acrylate polymers based uponFIGS. 6 and 7 and Table 3 do not appear to have been impacted in anymanner by the exposure to 20 wt. % HCl aqueous solution. This suggeststhat the polymers are suitable and stable for storing HCl aqueoussolution if encapsulated.

The diurethane dimethacrylate polymer is tested using DMA for storagemodulus and tan delta after no immersion, 24 hours of immersion and 2weeks of immersion in a 20 wt. % HCl aqueous solution at 25° C. toexaminer long-term exposure effects to aqueous 20 wt. % HCl solution.FIG. 8 is a graph showing traces for determined storage modulus ofdiurethane dimethacrylate after no immersion, 24 hours of immersion and2 weeks of immersion in a 20 wt. % HCl solution. FIG. 9 is a graphshowing traces for determined tan delta of diurethane dimethacrylateafter no immersion, 24 hours of immersion and 2 weeks of immersion in a20 wt. % HCl solution. The mechanical properties of the acrylate polymerbased upon FIGS. 8 and 9 do not appear to have been impacted in anymanner by the exposure to 20 wt. % HCl aqueous solution even up to 2weeks of exposure. This suggests that the polymers are suitable andstable for storing HCl aqueous solution if encapsulated. After two weeksof acid solution exposure, only a ˜0.5 wt. % gain was detectedsuggesting minimal penetration and swelling of the polymeric material.

Encapsulation of Acid Solution Using Double Emulsion Polymerization

A double emulsion polymerization technique is used to preparepolymer-encapsulated mineral acid solution (a polymerized shellencapsulating an aqueous HCl in the core of the capsule).

A urethane acrylate monomer (SARTOMER CN9013) is used to form thepolymer shell. A double emulsion (water-in-oil-in-water emulsion)polymerization is used with the monomer solution as the “oil” phase toform the liquid core/polymer shell capsules. Two solutions are preparedfor the first emulsion. The first solution is an organic solutioncontaining SARTOMER CN9013 as the monomer, chloroform, a lipophilicsurfactant (Span® 80) (Sigma-Aldrich #85548) and an initiator. For the10 wt. % HCl core solution, 1 wt. % of AIBN (Sigma-Aldrich #441090) isused as a thermal initiator. At 20 wt. % and 30 wt. % HCl core solution,a blend of DPO and 2-hydroxyl-2-methlypropiophenone is used as aphoto-initiator. For preparation of organic solution, the urethaneacrylate is dissolved into chloroform by mixing and applying low heat.The ratio of chloroform:CN9013 is in a range of about 2:1 to about 1:1by weight. SPAN 80 is introduced into the composition in a range of fromabout 1 wt. % to about 5 wt. % on a monomer solution basis. Once themonomer is dissolved into the organic solution and cooled, the initiatoris added to prevent premature polymerization. The initiator isintroduced in a range of from about 1 wt. % to about 3 wt. % on amonomer solution basis.

The second solution is a strong mineral acid solution containingdeionized water, a surfactant mixture (TWEEN® 80/Pluronic® F-127) andhydrochloric acid (HCl). TWEEN 80 (#P4780) and PLURONIC F-127 (#P2443)are both available from Sigma-Aldrich. The inner solution mainlycomprises deionized water that is mixed with 37 wt. % aqueous HCl.Optionally, polyvinyl alcohol (PVA) is introduced to the strong mineralacid solution to balance density with the monomer solution and tothicken the viscosity of the strong mineral acid solution. The amount,when added, is up to about 1 wt. % of the solution. The PVA has anaverage molecular weight (M_(w)) of 146,000-186,000 and is 87-89%hydrolyzed. A mixture of TWEEN 80/PLURONIC F-127 is the surfactantpackage for the strong acid aqueous solution. TWEEN 80 is typicallyprovided at about 1 wt. % of the deionized water, and PLURONIC F-127 isintroduced at about 0.2 wt. % of the solution. The resulting compositioncontains HCl in a range of from about 10 wt. % to about 30 wt. % of thesolution.

The second aqueous solution, which is the continuous phase of thesecondary emulsion, contains deionized water, the surfactant mixture(TWEEN 80/PLURONIC F-127) and optionally PVA. The surfactantconcentration is the same as the strong mineral acid solution. Whenincluded, the PVA concentration is higher—about 2 wt. % of thesolution—than the PVA concentration in the strong mineral acid solution.Optionally, the second aqueous solution includes hydrochloric acid in arange of from about 1 wt. % to about 20 wt. % of the solution.

To prepare the capsules, 1 mL of the aqueous acid solution is added to avial containing 10 mL of the organic solution. The combination isemulsified into a primary emulsion at high RPM for two minutes,dispersing the acid aqueous solution into the oil phase. The primaryemulsion (W/O) is added into the second aqueous solution. Thecombination of the primary emulsion and the second aqueous solution isemulsified into a secondary emulsion (W/O/W) at high RPM for twominutes, dispersing the primary emulsion into the second aqueous phase.This forms a three-phase liquid pre-capsule.

Once the W/O/W forms, the double emulsion is cured. Vigorously stirringon a hot plate is maintained for 5 minutes. For the blend ofphoto-initiators, the cure is conducted using an ELC-403 light curingsystem while preventing the pre-capsules from settling together. Whenusing the AIBN initiator, the hot plate is maintained at 65° C. for thecuring time. The cured polymer-encapsulated mineral acid solution arecollected, washed, scanned and imaged.

FIGS. 10 a-c are scanning electron micrographs (SEMs) of synthesizedpolymer-encapsulated mineral acid solution having 10 wt. % HCl core.FIG. 10 a is at 50 μm magnification. FIGS. 10 b and 10 c are at 20 μmmagnification.

FIGS. 11 a-c are scanning electron micrographs (SEMs) of synthesizedpolymer-encapsulated mineral acid solution having 20 wt. % HCl core.FIG. 11 a is at 100 μm magnification. FIG. 11 b is at 2 mm. FIG. 11 c isat 400 μm. The voids in the polymer shell wall shown in FIG. 11 a isbelieved to be pockets filled with mineral acid solution while theliquid monomer solution formed into the solid polymer shell. The polymershell has a smooth exterior from forming contacting the second aqueoussolution. FIGS. 12 a-b are optical microscopy images of the synthesizedpolymer-encapsulated mineral acid solution having 20 wt. % hCl core.Both FIG. 12 images are at 400 μm resolution.

FIGS. 13 a-c are scanning electron micrographs (SEMs) of synthesizedpolymer-encapsulated mineral acid solution having 30 wt. % HCl core.FIG. 13 a is at 400 μm magnification. FIG. 13 b is at 2 mm. FIG. 13 c isat 500 μm. Unlike the 10 wt. % and 20 wt. % HCl capsules, the 30 wt. %HCl capsules show exterior pitting. The pitting may be caused by osmoticpressure exerted by the aqueous continuous phase. FIGS. 14 a-b areoptical microscopy images of the synthesized polymer-encapsulatedmineral acid solution having 30 wt. % HCl core.

Effects of Brine on Polymer-Encapsulated Mineral Acid Solution

Polymer-encapsulated mineral acid solution having 20 wt. % HCl core anda polymer encapsulating shell made from poly(ethylene glycol)dimethacrylate (having an average PEG M_(n) of about 600) monomer aresynthesized using the double-emulsion W/O/W polymerization techniquepreviously described. The ratio of monomer solution:strong mineral acidsolution is about 1:1. The photo-initiator is about 2 wt % of the entiresystem. FIG. 15 is a SEM of the synthesized polymer-encapsulated mineralacid solution having 20 wt. % HCl core and a poly(PEG dimethacrylate)shell. The capsules at 500 μm resolution appear clean, smooth andwell-formed. FIGS. 16 a-b are SEMs of the synthesizedpolymer-encapsulated mineral acid solution having 20 wt. % HCl core anda poly(PEG dimethacrylate) shell after exposure to brine for 24 hours at25° C. The brine is an aqueous solution containing 4 wt. % NaCl and 1wt. % CaCl₂. Both SEMs show the same well-formed and smooth capsules.The capsules shown in FIGS. 16 a-b appear unaffected by the externalpresence of the brine—the capsules appear solid and have not externalpitting or deformities.

Effects of Temperature on Acid Release from Polymer-Encapsulated MineralAcid Solution

Polymer-encapsulated mineral acid solution having 20 wt. % HCl core anda polymer encapsulating shell made from 1,6-hexanediol diacrylatemonomer are synthesized using the double-emulsion W/O/W polymerizationtechnique previously described. Some acid solution loss occurred duringformation of the capsules. The pH of the original aqueous dispersioncontaining the capsules initially is determined to be around 1.65. FIG.17 is an optical microscopy image of the synthesized 1,6-hexanedioldiacrylate polymer-encapsulated mineral acid solution having 20 wt. %HCl core. FIG. 17 image is at 50 μm resolution. The T_(g) of the productpolyacrylate shell is about 43° C.

The release profiles at several temperatures are determined using thesynthesized 1,6-hexanediol diacrylate polymer-encapsulated mineral acidsolution having 20 wt. % HCl core. Twenty mL of the original aqueousdispersion is distributed into 7 centrifuge tubes. For 6 of the samples,the synthesized capsules are washed down from their original solutionwith deionized water (no surfactant or additives). The 7th sample is acontrol. When the synthesized capsules are cleansed of external acid (pHstrip of 6 or equivalent to deionized water), the washed synthesizedcapsules are suspended in fresh deionized water to form a suspensiontotaling 15 mL and introduced into a 20 mL scintillation vial. Thenumber of capsules per sample is about 6×10⁶ capsules in 15 mL in 1M ofNaCl.

Three heated samples are placed into an oil bath maintained at 60° C.while three samples are maintained at “room temperature” of about 20° C.Note that the heated samples are maintained at a temperature higher thanT_(g) for the polyacryate and the room temperature samples aremaintained at temperature below T_(g). During each of the pHmeasurements, the heated samples are extracted from their oil bath andplace in a bath at room temperature to increase the cooling process. AllpH measurements are taken at room temperature to avoid anytemperature-related drift in pH detection. The average pH for eachsample at each period of detection is determined. The results areplotted on FIG. 18.

FIG. 18 is a graph showing traces for determined average pH versus timefor 20 wt. % HCl core/poly(1,6-hexanediol diacrylate) shell capsulesbelow and above-T_(g) temperature. The graph demonstrates the controlledrelease of the acid solution on the basis of temperature. Providing anenvironment where the temperature is maintained greater than the T_(g)of the polymer results in a faster release of the encapsulated aqueousacid versus providing a sub-T_(g) temperature. Acid diffusion does occurat room temperature, but at a much slower rate. This indicates a viabletriggering mechanism for the polymer-encapsulated mineral acid solution(temperature elevation at or beyond T_(g)). The room temperature traceprovides evidence of long-term acid diffusion through the polymer shell,which can provide a reduced rate etching to a formation that maybeneficially maximize the depth of penetration for the strong mineralacid while in the polymer-encapsulated mineral acid solution.

Osmotic Balance in Water Phases for Double Emulsion Polymerization

FIGS. 13 a-c show an exterior polymer shell for capsules having a 30 wt.% HCl strong mineral acid solution in the core. The shell pitting may bethe result of an imbalance in osmotic pressure between the strongmineral acid solution, which has a high concentration of negative ionsfrom the disassociated strong acid, and the second aqueous solutionacting as the continuous phase of the double emulsion.

Three sets of polymer-encapsulated mineral acid solution aresynthesized, where the continuous phase of the double emulsion—thesecond aqueous solution—is modified in each in regards to the objectivein negative ion concentration (for strong acid addition) and for anionconcentration (salt) to determine processing conditions for formingpolymer-encapsulated mineral acid solution with strong mineral acidsolution cores. For all three samples, the strong mineral acid solutionand the second aqueous solution comprise solution of about 1 to about 2wt. % TWEEN 80 and about 0.2 wt. % PLURONIC F-127 surfactants, water andthe salt or acid materials and aqueous concentrations (Molarconcentration) as given in Table 4. For forming the polymer shell,1,6-hexanediol diacrylate monomer with about 2 wt. % SPAN 80 lipophilicsurfactant and about 3 wt. % DTBP photo-initiator are introduced andform the monomer solution. The double emulsion pre-capsules are formedas previously described. The capsules are cured using the ELC-403 lightcuring system as previously described.

TABLE 4 Osmotic balance Samples 1-3 with different concentrations ofcore and continuous phase materials. Core material Continuous phasematerial Sample No. (molarity) (molarity) Sample 1  HCl [6.12M] NaCl[2M] Sample 2 NaCl [5.7M]   NaCl [5.7M] Sample 3 HCl [5.7M] NaCl [5.7] 

The formed capsules for Sample 1 are shown in FIG. 19. The formedcapsules for Sample 2 are shown in FIG. 20. The formed capsules forSample 3 are shown in FIG. 21. FIG. 19 is a SEM of synthesizedpolymer-encapsulated mineral acid solution formed in an osmoticallyunbalanced system. FIG. 19 shows capsules where there is about a 4Mnegative ion/anion concentration difference between the strong mineralacid solution and the second aqueous solution. Large amounts of surfacepits are shown similar to FIGS. 13 a-c. Although not wanting to belimited by theory, it is believed that the more dilute second aqueoussolution pushed into and through the forming polymers shell. The poresprevent proper containment of the acid core. FIGS. 20 (NaCl interior andexterior) and 21 (HCl interior; NaCl exterior) show SEMs of synthesizedpolymer-encapsulated mineral acid solution formed in osmoticallybalanced system.

FIGS. 20 and 21 appear to indicate that adding salt (or acid) to alignthe outer and inner ion concentration of the two aqueous solutionsprovides osmotic balance to the process, which produces much fewer poresin the polymer shells. FIGS. 10 a-c and 11 a-c indicate that a perfectbalance is not required. Both sets of capsules formed under theconditions described for their formation—believed to be imbalanced—donot show a significant amount of surface pitting. Therefore, it isbelieved that some osmotic pressure imbalance—at least enough not toovercome the speed and strength of the forming polymer shell—between thetwo aqueous phases is acceptable.

The effect of temperature was tested on the osmotically-balancedcapsules with NaCl (Sample 2) and HCl in their cores (Sample 3). Sample2 acts as a “negative control”, while the Sample 3 capsules are splitinto two sets: a set that is warmed above the T_(g) of the polymer shell(80° C.) and the other maintained at room temperature (20° C.) similarto the procedure previously described to produce the results shown inFIG. 18 (cooling heated sample to room tempearture before taking pH).FIG. 18 is a graph showing traces for determined change in average pHversus time for osmotically-balanced HCl [5.7M] core/poly(1,6-hexanedioldiacrylate) shell capsules below and above-T_(g) temperature and forsimilarly osmotically-balanced NaCl [5.7M] capsules. Similar to theresults given in FIG. 18, the capsules elevated beyond T_(g) degradequickly and release their acid solution core into the surroundingsolution, whereas the capsules at room temperature more gently diffuseafter an initial release of acid solution from thinner-walled capsules.Each data point shown in FIG. 22 is an average of two measurements takenat each time for each temperature exposure. The heated capsules weretaken to 80° C. and then cooled prior to measuring. All pH measurementswere taken at room temperature to ensure minimal variation betweensamples. This helps to confirm that introduction into the well bore at asub-Tg temperature and permitting teh formation to elevate tehtemperature through natural processes can permit the deep introductionof capsules into fissures and fractures before the polymer shellthermally degrades and releases its acid core for a majority of thecapsules. The release of acid solution from capsules that do so atsub-T_(g) temperatures may assist in “clearing the path” for other, moreintact and resilient capsules to penetrate deep into the formation,where they can then warm and thermally degrade deeper in the formation.

Negative Example—Attempted Encapsulation of HCl Acid Solution UsingDouble Emulsion Polymerization with Epoxy Functional End Group Monomers

Free-radical chain polymerization appears to be unaffected by the strongacid present in the strong mineral acid solution of the core of thecapsule. Step polymerization, such as how epoxy functional end groupmonomers react with one another and comonomers, however, appears to begreatly affected by the presence of the strong mineral acid.

The procedure as given for forming the urethane acrylate monomer(SARTOMER CN9013) polymers shell-based capsules (See “Encapsulation ofacid solution using double emulsion polymerization” supra) is used withminor differences to account for the epoxy experiment. The organicsolution for forming the primary emulsion contains EPON Resin 862(Momentive Specialty Chemicals; Houston, Tex.), which is a diglycidylether of Bisphenol F, for the monomer instead of an acrylate. Thecurative in the monomer solution is Amicure PACM (Air Products &Chemicals; Allentown, Pa.), which is a thermally-activated curativeagent for epoxy systems. The monomer solution comprises 11.445 gramsEPON Resin 862, 3.55 grams Americure PACM curative, 0.45 grams SPAN 80,and 11.49 grams chloroform. The strong mineral acid solution for formingthe primary emulsion is the same except it only contains 1 wt. % HClinstead of 10-30 wt. % HCl and TWEEN® 20 (Sigma-Aldrich P2287) issubstituted for TWEEN 80. The strong mineral acid solution is acombination of 200 μL of 37 wt. % HCl and 924 μL of a solutioncomprising 36 mg of PVA, 11 mg of TWEEN 20, 23 mg of NaCl and 2.625 mLof deionized water. The second aqueous solution is the same aspreviously described. With the low HCl concentration, there is no needfor compensating for any difference in osmotic effects while the capsuleforms during secondary emulsification and curing. The pre-capsule doubleemulsion is cured thermally at 50° C. for 2 hours to form the curedepoxy polymer-encapsulated mineral acid solution.

FIGS. 23 a-c are optical microscopy images of the synthesized epoxypolymer-encapsulated mineral acid solution having 1 wt. % HCl cores.During initial examination, there appears to be formed capsules havingepoxy shells and acid solution cores. The resolution of FIGS. 23 a and bis 100 μm, the resolution of FIG. 23 c is 50 μm. FIGS. 24 a-b are SEMsimages of synthesized epoxy polymer-encapsulated mineral acid solutionand resultant salts. Before or during the attempt to analyze thecapsules using SEM, the epoxy-based shell capsules destabilized andcollapsed, resulting in the formation of salt capsules in the shapes ofthe liquid core. FIG. 24 a is at 100 μm resolution; FIG. 24 b is at 20μm resolution.

A second attempt to synthesize epoxy polymer-encapsulated mineral acidsolution uses neutral water as the core material for forming a firstcapsule using the EPON Resin 862. A second capsule with a 1 wt. %aqueous mineral acid solution core is formed using the same procedure aspreviously given. Both sets of capsules are examined using SEMmicrography.

FIGS. 25 a-b are SEMs of the synthesized epoxy polymer-encapsulatedneutral water capsules. The neutral water core capsules appear to have asmooth external surface without and well-formed. The resolution of FIGS.25 a-b is 500 μm. FIGS. 26 a-b are SEMs of resultant salts fromattempting to synthesize epoxy polymer-encapsulated mineral acidsolution. After encapsulation of the 1 wt. % HCl, it appears that theformed capsules once again are not stable. FIG. 26 show only resultantsalt crystals and no epoxy shell capsules. Although not wanting to bebound by theory, the proton donation effect of the strong mineral acidlikely interferes with the step polymerization reaction of the epoxy endgroups by cleaving the epoxy ring. This results in a non-fullyincorporated shell that collapses easily and releases the core material.

What is claimed is:
 1. A method for forming a polymer-encapsulatedmineral acid solution, the method comprising the steps of: introducing astrong mineral acid solution to a monomer solution such that a primaryemulsion forms, where the primary emulsion is a water-in-oil (W/O) typeemulsion, where the strong mineral acid solution comprises a strongmineral acid that is in a range of from greater than 0 wt. % to 30 wt. %of the strong mineral acid solution, where the monomer solutioncomprises a monomer and a free-radical initiator, and where the monomeris hydrophobic and is operable to polymerize upon initiation of afree-radical chain polymerization reaction; introducing the primaryemulsion to a second aqueous solution such that a secondary emulsionforms, where the secondary emulsion is a water-in-oil-in-water (W/O/W)type double emulsion; and converting the free-radical initiator in themonomer solution such that a free-radical chain polymerization reactionis initiated in the monomer solution, the polymerized shell forms, andthe secondary emulsion converts into the polymer-encapsulated mineralacid solution, where the polymerized shell encapsulates the strongmineral acid solution and prevents interaction with the strong mineralacid solution until the polymerized shell degrades, and where the strongmineral acid solution does not degrade the polymerized shell.
 2. Themethod of claim 1 where the strong mineral acid is selected from thegroup consisting of hydrochloric acid (HCl), hydrofluoric acid (HF),sulfuric acid (H₂SO₄), nitric acid (HNO₃) and combinations thereof. 3.The method of claim 1 where the monomer solution comprises a monomerhaving acrylate end group functionality.
 4. The method of claim 1 wherethe monomer solution comprises a monomer having methacrylate end groupfunctionality.
 5. The method of claim 1 where the monomer solution has adensity that is not substantially different than the density of thestrong mineral acid solution.
 6. The method of claim 1 where the monomersolution has a viscosity that is not substantially different than theviscosity of the strong mineral acid solution.
 7. The method of claim 1where the free-radical initiator is a photo-initiator.
 8. The method ofclaim 1 where the secondary aqueous solution further comprises an acid.9. The method of claim 8 where the secondary aqueous solution has anegative ion concentration that is not substantially different than thenegative ion concentration of the strong mineral acid solution.
 10. Themethod of claim 1 where the secondary aqueous solution further comprisesa salt.
 11. The method of claim 10 where the secondary aqueous solutionhas a negative anion concentration that is not substantially differentthan the negative ion concentration of the strong mineral acid solution.12. The method of claim 1 where converting the free-radical initiator inthe monomer solution occurs by introducing electromagnetic energy intothe secondary emulsion.
 13. A polymer-encapsulated mineral acid solutioncomprising: a strong mineral acid solution where the strong mineral acidis in a range of from greater than 0 wt. % to about 30 wt. % of thesolution; and a polymerized shell that encapsulates the strong mineralacid solution such that the polymerized shell encapsulates and preventsinteraction with the strong mineral acid solution until the polymerizedshell degrades, where the strong mineral acid solution does not degradethe polymerized shell.
 14. The capsule of claim 13 where the strongmineral acid is selected from the group consisting of hydrochloric acid(HCl), hydrofluoric acid (HF), sulfuric acid (H₂SO₄), nitric acid (HNO₃)and combinations thereof.
 15. The capsule of claim 13 where thepolymerized shell comprises a polyacrylate homopolymer.
 16. The capsuleof claim 13 where the polymerized shell comprises a polymethacrylatehomopolymer.
 17. The capsule of claim 13 where the polymerized shellcomprises polymerized monomer having acrylate end group functionality.18. The capsule of claim 13 where the polymerized shell comprisespolymerized monomer having methacrylate end group functionality.
 19. Thecapsule of claim 13 where the polymerized shell is the resultant of afree-radical chain polymerization reaction.
 20. The capsule of claim 13where the polymerized shell has a glass transition temperature (T_(g))and the T_(g) is in a range of from 43° C. to 151° C.
 21. The capsule ofclaim 13 where the polymerized shell does not have a glass transitiontemperature (T_(g)).
 22. A polymer-encapsulated mineral acid solutionsuspension comprising: a polymer-encapsulated mineral acid solution,comprising: a strong mineral acid solution where the acid is in a rangeof from greater than 0 wt. % to about 30 wt. % of the solution, and apolymerized shell that encapsulates the strong mineral acid solutionsuch that the polymerized shell contains the strong mineral acidsolution and prevents interaction with the strong mineral acid solutionuntil the polymerized shell degrades, where the strong mineral acidsolution does not degrade the polymerized shell; and a suspension fluid.23. The solution of claim 22 where the polymerized shell has a glasstransition temperature (T_(g)) and where the suspension fluid has atemperature that is less than T_(g).
 24. The solution of claim 22 wherethe suspension fluid is a brine.
 25. A method of stimulating ahydrocarbon-bearing formation using a polymer-encapsulated mineral acidsolution, the method comprising: introducing a polymer-encapsulatedmineral acid solution suspension into a fissure traversing thehydrocarbon-bearing formation to be stimulated; and maintaining thepolymer-encapsulated mineral acid solution within the fissure such thatthe polymer shell degrades within the fissure, the strong mineral acidsolution is released into the fissure of the hydrocarbon-bearingformation, and the hydrocarbon-bearing formation is stimulated, wherethe polymer-encapsulated mineral acid solution suspension comprises asuspension fluid and a polymer-encapsulated mineral acid solution, wherethe polymer-encapsulated mineral acid solution comprises: a strongmineral acid solution where the acid is in a range of from greater than0 wt. % to about 30 wt. % of the solution, and a polymerized shell thatencapsulates the strong mineral acid solution such that the polymerizedshell contains the strong mineral acid solution and prevents interactionwith the strong mineral acid solution until the polymerized shelldegrades, where the strong mineral acid solution does not degrade thepolymerized shell; and where the well bore is defined by a well borewall and traverses the hydrocarbon-bearing formation, where the face isa portion of the well bore wall associated with and operable to providefluid communication between the hydrocarbon-bearing formation and thewell bore, and where the fissure in the hydrocarbon-bearing formation isaccessible through the face.
 26. The method of claim 25 where thepolymerized shell has a glass transition temperature (T_(g)) and wherethe introduced polymer-encapsulated mineral acid solution suspension hasa temperature that is less than T_(g).